This document summarizes the benchmark performance of geothermal heat pump (GHP) systems installed at four new elementary schools in Lincoln, Nebraska. Vertical-bore GHP systems were installed in 1995 and have since provided heating and cooling with utility costs nearly half of other schools in the district. Performance data shows the GHP schools use on average 26% less source energy per square foot than other new schools in the district with similar ventilation loads. While variations exist among the four GHP schools, together they still consume less source energy than 70% of all schools in the district.

1. SE-99-20-03
Benchmark for Performance: Geothermal
Applications in Lincoln Public
Schools
John A. Shonder Michaela A. Martin, P.E. Terry R. Sharp, P.E.
Member ASHRAE Member ASHRAE Member ASHRAE
David J. Durfee Patrick J. Hughes, P.E.
Member ASHRAE
ABSTRACT across the nation. Vertical-bore, ground-coupled GHP
systems were installed at four new elementary schools
Vertical-bore, geothermal heat pumps (GHPs) have located in Lincoln, Nebraska in 1995 ͑Figure 1͒. The schools
been providing heating and cooling to four new elementary have identical floor plans, each with 69,000 square feet of
schools located in Lincoln, Nebraska since 1995. According area dedicated to classrooms, offices, meeting rooms, a
to representatives of the local utility and school district, the cafeteria, and a gymnasium ͑Figure 2͒. Approximately 500
systems are providing a comfortable, complaint-free environ- students attend each school. The performance of these instal-
ment with utility costs that are nearly half of that of other lations is well-documented by electric and gas utility data ͑in
schools in the district. Performance data collected from 15-minute and monthly intervals͒ and 10-minute energy
on-site energy management systems and district billing and management system ͑EMS͒ data. Additionally, the situation
utility records for all fifty schools in the Lincoln district indi- at Lincoln is unique in that the district maintains records on
cate that only five consume less energy than the best facility design, energy performance, and maintenance activi-
performing GHP school; however, these five cool less than ties for all facilities within the district. This study is part of a
10% of their total floor area, while the GHP schools cool thorough effort to review the design and performance of the
100% of their floor area. When compared to other new GHP systems located at Lincoln Public Schools ͑LPS͒, and to
schools (with similar ventilation loads), the GHP schools compare the performance of these schools to others within
used approximately 26% less source energy per square foot the district.
of floor area. Variations in annual energy performance are
evident amongst the four GHP schools, however, together
they still consume less source energy than 70% of all schools SYSTEM DESIGN
in the district. These variations are most likely due to opera- The design of the schools’ mechanical systems was the
tional differences rather than installed equipment, building result of a collaborative effort between the engineer, the
orientation, or environmental (bore field) conditions. school district, and the local electrical utility ͑Bantam and
Benson 1995͒. Life cycle costs for five alternative designs
INTRODUCTION were analyzed using energy consumption and demand
profiles estimated from simulations performed with a
Currently, there are over 400 installations of geothermal commercially-available software package, operating costs
heat pump ͑GHPs͒ systems located at public school facilities estimated using utility rate schedules, and assumptions about
John A. Shonder is a staff member, Michaela A. Martin is a staff engineer, and Terry R. Sharp is a staff engineer at Oak Ridge National
Laboratory, Oak Ridge, TN. David J. Durfee is a research associate at the University of Tennessee, Knoxville, TN. Patrick J. Hughes is
a program manager at Oak Ridge National Laboratory, Oak Ridge, TN.
THIS PREPRINT IS FOR DISCUSSION PURPOSES ONLY, FOR INCLUSION IN ASHRAE TRANSACTIONS 1999, V. 105, Pt. 2. Not to be reprinted in whole or
in part without written permission of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA 30329.
Opinions, findings, conclusions, or recommendations expressed in this paper are those of the author(s) and do not necessarily reflect the views of ASHRAE. Written
questions and comments regarding this paper should be received at ASHRAE no later than July 7, 1999.

2. schools. Because the heat pumps at the Campbell and Maxey
schools are from one manufacturer, and those at Cavett and
Roper from another, there is some difference in the nominal
capacities installed, but this difference is minimal and is not
expected to alter building performance in any significant
manner.
The schools were designed to meet ASHRAE Standard
62-1989, which calls for at least 15 cfm of fresh air per
person. At each school, pre-conditioned outdoor air is
provided to classroom and office heat pump units by two
Figure 1 GHP elementary school, built in 1995 and located in
Lincoln, Nebraska. nominal 15-ton heat pumps ͑with two 7.5 ton compressors
each͒, located within a mechanical room. Each large unit
operates on 100% outdoor air and feeds this preconditioned
maintenance and equipment replacement intervals and air into local heat pumps through two central ducts running
installed costs estimated with a commercially-available cost along the schools’ main corridors. Additional outdoor air is
estimating guide. These designs included: a variable air provided to assembly areas, such as the multi-purpose
volume ͑VAV͒ system with air-cooled chillers, a VAV cafeteria and gymnasium, by a nominal 10-ton unit operating
system with water-cooled chillers, gas-fired absorption on 40% outdoor air and a nominal 4.5-ton unit with 45%
chillers, vertical GHPs, and water-loop heat pumps. Various outdoor air. In all outdoor air units, preheat is provided by a
time-consuming adjustments were needed during the feasi- hot water coil when ambient temperatures fall below 40 °F.
bility study in order to overcome shortcomings in GHP repre- Hot water is also supplied to terminal units located in vesti-
sentations in available software tools. Considering capital bules and other perimeter areas. Hot water is produced by
costs and likely operating and maintenance costs, vertical gas-fired boilers, 4 per school, each with a capacity of
GHPs were determined to be the best alternative. 330,000 BTU/hr.
Because their annual operating schedules may fluctuate The remaining heat pumps, ranging in size from 1.4 to
from year to year, heating and cooling loads were estimated 4.5 nominal tons, serve individual zones: classrooms, offices,
assuming full-year operation of the facility. According to the and common group study areas. For the most part, these units
loads calculated during the design process, under full-year are located above the central corridors outside the zones they
operation, the building was expected to be dominated by serve, and are easily accessible to maintenance personnel.
cooling loads. The total block cooling load for the 32-zone At all four schools, the heat pumps absorb and reject
building was estimated to be 150 tons, while the peak block heat through a common loop to a bore field consisting of 120
heating load was 940,000 Btu/hr ͑Kavanaugh 1994͒. Full- bores arranged in a 12 by 10 pattern with 20 foot spacing.
load heating and cooling hours were estimated at 500 and Figure 3 presents a site plan including a layout of the bore
750, respectively. field, which is located under the school’s soccer field. The
Fifty-four heat pumps of various sizes meet the heating bores are 240 feet deep, with diameters of 4 1/4 inches on the
and cooling loads at each of the schools. Table 1 presents the lower 220 feet, and 6 inches on the top 20 feet. Fine gravel
sizes and quantities of each heat pump installed at the four pack was used to back fill the boreholes to within 10 feet of
the surface, at which point a bentonite plug was provided in
order to prevent groundwater contamination from the surface
͑in compliance with Nebraska State regulations, where a
surface plug is acceptable if only one aquifer is penetrated͒.
Fine gravel pack was judged to provide adequate pipe
thermal contact because the static water level was considered
to be between 20 to 40 feet ͑and in most instances closer to
20 feet͒. The loops consist of thermally-fused, 1-inch diam-
eter high density polyethylene piping ͑SDR-11͒. At each
school, approximately 10,000 gallons of water containing
22% propylene glycol are circulated through the system by a
30 horsepower pump controlled by a variable-frequency
drive ͑VFD͒.
ENERGY PERFORMANCE
Total annual energy consumption per square foot is
presented for each school in Table 2. Because the schools
Figure 2 Floor plan for GHP schools in Lincoln, Nebraska consume both natural gas and electricity, the appropriate
(69,000 ft2). form of energy consumption used in benchmarking analyses
2 SE-99-20-03

3. TABLE 1
Schedule of Water to Air GHP Units Located at Four Lincoln Elementary Schools
Campbell and Maxey GHP Units Cavett and Roper GHP Units
͑Manufacturer A͒ ͑Manufacturer B͒
Unit Quantity Tons/Unit Total Capacity Unit Quantity Tons/Unit Total Capacity
HP1H.1 1 1.4 1.4 HP1H.1 1 1.4 1.4
HP1H.2 4 1.4 5.6 HP1H.2 4 1.4 5.6
HP1V 1 1.4 1.4 HP1V 1 1.4 1.4
HP2H 4 2.0 8.0 HP2H 4 1.8 7.2
HP2V 2 2.0 4.0 HP2V 2 1.8 3.6
HP3H.1 1 2.0 2.0 HP3H.1 1 2.3 2.3
HP4H.1 34 3.5 119.0 HP4H.1 34 3.0 102
HP4H.2 1 3.5 3.5 HP4H.2 1 3.2 3.2
HP4V 1 3.5 3.5 HP4V 1 3.0 3.0
HP5V.1 1 4.5 4.5 HP5V.1 1 4.6 4.6
HP5V.2 1 4.5 4.5 HP5V.2 1 4.6 4.6
HP6V 1 10.0 10.0 HP6V 1 10.0 10.0
HP7V 2 15.0 30.0 HP7V 2 16.0 32.0
Total 54 197.4 Total 54 180.9
is source energy. This format accounts for the average effi- expansions, and HVAC system types and ages were
ciency of producing electricity from fossil fuel and delivering collected. Using multiple sources of information ͑utility
it to the site, assumed to be 33%. With an average annual account data, 1996 and 1997 LPS billing records, facility
energy consumption of 85.3 kBtu/ft2, Campbell school is the reports, and equipment inventories͒ a rigorous verification of
lowest consumer of energy amongst the four GHP schools. the energy and building characteristics information resulted
Maximum annual consumption ͑at 102.7 kBtu/ft2͒ occurred in a qualified data set of 50 schools.
at Cavett in 1997. Energy costs per square foot for the four GHP schools
In order to study the performance of the Lincoln GHP have already been reported to be about half that of other
schools in comparison with the rest of the schools in the schools in the Lincoln District ͑Feuerbach and Bantam
district, energy consumption and cost data were collected for 1998͒. Figure 4 compares the schools’ average annual source
all schools. This included 37 elementary schools, 11 middle energy use per square foot with that of other schools in the
schools, and 5 high schools. Additionally, physical charac- Lincoln school district. The data indicate that the GHP
teristics on each school, such as floor area, age and number of schools are exceptionally low energy users. Campbell school
is the lowest: only 12% of the schools in the district use less
energy per square foot. Even for the highest average energy
user of the four, Maxey school, only 30% of the schools in
TABLE 2
Annual Energy Consumption „1996, 1997, and
average…, per Square Foot of Floor Area, for all
Four Lincoln GHP Schools
1996 Source 1997 Source Average Annual
Energy Energy Source Energy
Consumption Consumption Consumption
School „kBtu/ft2… „kBtu/ft2… „kBtu/ft2…
Campbell 84.7 85.8 85.3
Roper 95.9 93.5 94.8
Cavett 90 102.7 96.4
Figure 3 Site plan for Lincoln GHP schools, including bore field
layout. Maxey 101.4 96.1 98.2
SE-99-20-03 3

4. Figure 4 Distribution of average annual source energy consump-
Figure 5 Distribution of average annual source energy consump-
tion for K-12 schools located in the Lincoln Public
tion for K-12 schools located in EIA Census Division 4.
School District (Lincoln, NE). Schools that provide
space cooling to over 90% of their total floor area are
identified using solid data markers. Three schools built
in the 1990’s, with conventional HVAC systems, are also
identified. Iowa, Missouri, and Kansas.1 It is evident that 37% of
schools in the census region use less energy than Campbell
school, while 49% use less energy than Maxey school.
the district use less energy per square foot. These numbers Further investigation into the EIA data suggests that 49% of
are even more impressive when it is considered that most of the Region 4 schools cool less than 35% of their total floor
the schools ranking below the GHP schools cool less than area. Conversely, only 30% of the schools cool 100% of their
15% of their total floor area ͑only 2 cool between 70% and buildings.
90% of their floor area͒. In fact, 48% of the schools in the As Figure 6 indicates, the energy use of the Lincoln
Lincoln district cool less than 25% of their respective floor GHP schools compares favorably with the energy use of all
areas. Only seven schools ͑including the four GHP schools͒ elementary schools in the U.S.2 Based on data collected from
air condition 100% of their floorspace. The average source K-12 schools from across the United States ͑EIA, 1995͒, only
energy use for schools that air condition less than 25% of 27% use less energy than the Campbell school, while 37%
their floorspace is 100.9 kBTU/ft2; for schools that air condi- use less energy than the Maxey school. On a national basis,
tion more than 70% of their floorspace, the average source 49% of the schools cool 100% of their floor area, while 33%
energy use is 120.4 kBTU/ft2. The average annual source cool less than 20% of their floor area.
energy consumed by each of the four GHP schools is The variation in consumption behavior between Camp-
93.7 kBTU/ft2. bell and the other three schools is not insignificant. The
In addition to floor space cooled, older schools do not drivers behind this variation in performance are more likely
deliver the same levels of outdoor air per person. Figure 4 due to operational differences than installed equipment, or
also provides a comparison of source energy use for all facility or environmental conditions. For instance, the heat
Lincoln schools built in the 1990’s. This is useful because pumps used in Campbell and Maxey were produced by a
both the percent of floorspace cooled and ventilation air different manufacturer than those used in Cavett and Roper,
delivered are similar for the new schools. The non-GHP so the difference does not correlate with manufacturer. Addi-
schools rely on either air-cooled chillers or air-cooled tionally, the difference does not seem to correlate to building
condensers to cool 79%–100% of their floor area. On orientation or number of portable buildings. Cavett and
average, the GHP schools use 26% less source energy per Roper face east, Maxey faces west, and Campbell faces
square foot, per year, than the non-GHP schools. south; Cavett and Roper each have two portable classroom
Using statistical representations of census data collected buildings on-site as well. Finally, no strong correlation can be
by the Energy Information Administration ͑EIA͒, Figure 5 drawn from the small amount of data available on site soil
compares the GHP schools’ average annual source energy conditions. Recent in-situ tests conducted at Campbell and
use per square foot with the energy use of elementary schools Maxey estimated thermal conductivities at 1.20
located in U.S. Census Region 4 ͑EIA, 1995͒. This region Ϯ0.06 Btu/hr-ft-°F and 1.35Ϯ0.15 Btu/hr-ft-°F, respectively.
includes Nebraska, North Dakota, South Dakota, Minnesota, This suggests that bore field heat transfer ability at Maxey is
1
Census Division 4 includes the states of Nebraska, North Dakota, South Dakota, Minnesota, Iowa, Missouri, and Kansas. EIA data for Division 4 is based
on a weighted statistical sample of 21 schools estimated by EIA to represent the entire building population of the census division ͑in this case 9,787 schools͒.
2
EIA data for the entire U.S. is based on a weighted statistical sample of 449 schools estimated by EIA to represent the entire building population of the U.S.
͑162,689 schools͒.
4 SE-99-20-03

5. 42 °F is most likely due to a number of factors: high occu-
pancy, high interior lighting requirements, other interior
loads such as computers and closed-circuit classroom moni-
tors, and of course the existence of the outdoor air preheat
coils, which are heated by natural gas. In comparing param-
eters from one school to the next, only E o seems to partially
explain lower energy consumption at Campbell school,
͑without examination of corresponding gas consumption
data, which was not available͒. E o for Campbell is 698.4
kWh/day, which is 5%–9% lower than the other three GHP
schools.
Data for the four schools on non-HVAC-related daily
electrical energy use ͑for lights and other electrical equip-
ment͒ on school days are presented in Figure 8. The daily
Figure 6 Distribution of average annual source energy consump- electrical use is seen to be fairly constant, with a slight nega-
tion for all U.S. K-12 schools (from EIA census data).
Data from 300 to 1,500 kBtu/sq. ft-yr were omitted for
tive correlation with outdoor air temperature. Table 4
clarity. presents the average daily non-HVAC electrical consump-
tion, for each school. Campbell is the lowest non-HVAC
consumer at 922 kWh per day, while Roper is the highest, at
higher than at Campbell; however, the overlap of the confi-
1,171 kWh per day.
dence intervals makes it difficult to draw any solid conclu-
Because the EMS system was not programmed to
sions. Additionally, boring logs prepared for the geotechnical
collect natural gas use, the only gas data available are
survey indicated that moisture content ͑using 120 ft bores͒
monthly billing records. In Figure 9, monthly ͑1996͒ natural
was highest for the soil samples collected from Campbell and
gas use in therms is plotted against heating degree days per
Cavett schools.
month, with a positive trend in consumption with increasing
In order to more closely examine the reasons for varia-
degree day. Annual gas consumption in 1996 was highest at
tion in performance of the four GHP schools, daily electrical
Roper ͑15,927 therms/year͒ and lowest at Cavett ͑10,916
consumption was studied as a function of daily average
therms/year͒. This represents a nearly 45% difference in
outdoor air temperature. This exercise was first attempted for
natural gas consumption. These differences are most likely
the entire data set, but there appear to be significant differ-
due to variations in set point and setback control by the
ences in the way the four schools are operated on days when
energy management systems.
classes are not in session. It may be that temperature setback
functions are not always implemented on weekends and holi-
days, especially during colder winter months; or that the EMS DATA
schools are used for other purposes ͑adult education, commu-
nity meetings, etc.͒ during these periods. Because of these The energy management system located at each school
variations it was decided to analyze the electrical energy use is capable of collecting 10-minute data for numerous
data for official schooldays only. Figure 7 presents the 1996 controller channels. While EMS data were collected for all
and 1997 daily electrical use for the HVAC systems ͑all schools from system initialization in 1995 through 1997, the
components͒ of each school vs. the average temperature on data set collected in 1996 for Maxey was the most complete
that day, for regular schooldays only. All four schools show and reliable ͑Carlson 1998͒. A review of this data provides
a typical profile for buildings heated and cooled by heat some additional insight into the operation and performance of
pumps. Daily electrical energy use for HVAC, E, for each Maxey’s GHP system.
school was fit to a function of daily average temperature, As mentioned earlier, the GHP systems at Lincoln were
ͭ
T a v g , of the form: designed to satisfy the cooling-dominated loads expected for
E o ϩm H ͑ T a v g ϪT H ͒ ͕ T a v g ՞T H ͖ a schedule based on full-year operation. While some activi-
Eϭ E o ͕ T H рT a v g рT C ͖ ties do occur during the summer months, Maxey generally
E o ϩm C ͑ T a v g ϪT C ͒ ͕ T a v g ՝T C ͖ operates on a traditional September through June schedule.
As a result of this schedule shift, the actual loads exhibited by
where E o is the baseline daily electrical energy use ͑i.e. that the building slightly favor heating. This is verified by loop
portion of the energy use which is not dependent on outdoor flow rate and temperature data collected by the EMS. In
temperature͒, T H and T C are the electrical heating and 1996, the total amount of energy rejected by the ground loop
cooling balance temperatures respectively, and m H and m C was 880 MMBtu, while the total energy absorbed was 1,004
are the slopes of the daily electrical energy use vs. daily MMBtu, a 13% difference. In contrast, the original design
temperature lines in heating and cooling modes, respectively. documents indicate that, on a full-year operating basis,
The parameters for each school are presented in Table 3. annual cooling loads were expected to be nearly three times
The average electric heating balance point temperature of the size of annual heating loads.
SE-99-20-03 5

6. Figure 7 Daily electrical energy use by all HVAC equipment for days when classes are in session (1996–1997).
As a consequence of traditional September through June EMS data, loop temperatures remain below 65 °F for nearly
operation, the loop water temperatures entering the heat 90% of the year. ͑This distribution is also impacted by the
pumps never approach the cooling design temperature of use of night time setback, during which EWTs remain rela-
90 °F. Figure 10 illustrates the distribution of entering water tively constant.͒ The maximum recorded value of EWT was
temperatures ͑EWTs͒ throughout the year. According to the 78 °F, while the minimum was 45 °F.
A 30 hp pump controlled by a variable-frequency drive
TABLE 3 varies the loop water flow to maintain supply/return differ-
Curve Fit Parameters for Daily HVAC Electrical ential pressure as 2-way, motorized, ball valves open and
Energy Use for Four Lincoln GHP Schools close with compressor cycles at each heat pump. According
to test and balance reports and EMS data, the total loop flow
Curve fit
Parameter Maxey Campbell Cavett Roper through the 54 heat pumps, with all 2-way valves open, was
552 gpm. This corresponds to approximately 2.8 gpm per
Eo 744.7 698.4 767.5 740.2 installed ton of capacity, rather than the maximum percent of
TH 43.10 42.61 43.32 39.26 installed capacity that operates simultaneous at peak block
load ͑about 86.3% of installed heating capacity and 56.8% of
TC 69.95 74.38 75.10 66.10
installed cooling capacity͒. Figure 11 shows the distribution
mH Ϫ29.09 Ϫ21.17 Ϫ25.85 Ϫ21.17
of loop flow rates in 1996. The maximum flow rate recorded
mC 60.20 119.1 183.59 23.97 was 552 gpm. Loop flow rates remain below 300 gpm for
6 SE-99-20-03

7. Figure 8 Daily electrical energy use by all non-HVAC equipment (lights, computers, etc.) for days when classes are in session (1996–
1997).
78% of the year, and below 150 gpm for 50% of the year. many of the motorized 2-way valves has failed causing vari-
Again, a night time system setback impacts this distribution able flow operations to be halted while valves are left in the
with minimal after-hour flow rates and larger flow rates open position. EMS data confirms that flow rates for Maxey
during recovery periods. Clearly, the potential for energy remained constant after April 1997.
savings is significant with the use of the variable-frequency
drive. Unfortunately, Lincoln reports that the packing on
CONCLUSIONS
TABLE 4 Energy data collected over a two-year period indicate
Average Daily Building-Related „non-HVAC… that the four new elementary schools located in Lincoln,
Electrical Energy Consumption by Four Lincoln Nebraska, out perform others in the school district. Of fifty
GHP Schools schools studied, only five consume less energy than the best
performing GHP school; however, these five cool less than
Maxey Campbell Cavett Roper 10% of their total floor area, while the GHP schools cool
100% of their floor area. When compared to other new
Average Daily
Bldg-Related 1,106 kWh 922 kWh 1,057 kWh 1,171 kWh schools ͑with similar ventilation loads͒, the GHP schools
Electrical used approximately 26% less source energy per square foot
Consumption of floor area. A variation in performance between the four
SE-99-20-03 7

8. Figure 9 Monthly natural gas use versus monthly heating degree days for GHP schools (1996).
Figure 10 Distribution of 1996 heat pump entering water
temperatures (EWT), as recorded by the Maxey EMS. Figure 11 Distribution of 1996 loop flow rates, as recorded by the
Total hours of operation were 8,760. Maxey EMS. Total hours of operation were 8,760.
8 SE-99-20-03

9. GHP schools is evident, and is most likely due to differences REFERENCES
in occupancy and equipment loading and scheduling. Bantam, D.D. and Benson, S.J. 1995. A public utility strategy
The data collected from the GHP schools is presently
for implementing and monitoring a ground-coupled heat
being used to study the ability of simulation tools ͑DOE-2,
pump system in a public school. APPA Energy/
TRNSYS͒ to model GHP systems with vertical borehole heat
exchangers. Following a validation of these models, a side- Customer Services and Communications Workshop,
by-side comparison of vertical-bore GHPs with conventional Kansas City, MO.
heating and cooling systems will be performed using cali- Carlson, S. 1998. Lincoln GHP EMS analysis and data notes.
brated loads taken from the Lincoln GHP schools. Cazenovia, NY: CDH Energy.
Energy Information Administration ͑EIA͒. 1995. Commer-
ACKNOWLEDGMENTS cial buildings energy consumption and expenditures
1992. DOE/EIA-0318͑92͒. Washington, D.C.: Depart-
The authors wish to recognize the following representa-
tives of the local utilities, school district, and engineering ment of Energy.
firm who were directly involved with the project and who Feuerbach, R.S. and D. Bantam. 1998. Going underground.
have generously shared their time, data, and experience: Engineered Systems.
Byron Bakenhus, Doug Bantam, Scott Benson, Patrick Kavanaugh, S. 1994. Ground-coupled heat pump system for
Decker, Ron Feuerbach, Alfred Hopp, Larry Hennings, Tim 150 ton school in Lincoln, Nebraska. Professional Corre-
Pratt, and Bill Lucke. spondence to Lincoln Electric System.
SE-99-20-03 9